This application is a 371 of PCT/GB00/02613, filed Jul. 7, 2000.
The present invention concerns a process for the preparation of 3,5-dichloropyridine. 3,5-Dichloropyridine is used as an intermediate in the chemical industry (especially in the agrochemical industry).
The present invention provides a process for the preparation of 3,5-dichloropyridine comprising reacting a trichloropyridine, a tetrachloropyridine or pentachloropyridine with zinc metal, in the presence of an acidic compound and at a temperature in the range 50-120xc2x0 C.
Trichloropyridines include 2,3,5-trichloropyridine. Tetrachloropyridine is 2,3,4,5- or 2,3,5,6-tetrachloropyridine.
It is preferred that the process of the present invention is conducted in the presence of a protic solvent, such as water or an alcohol (for example sec-butanol). Also, the process of the present invention can be conducted in the presence of a mixture of solvents (such as a mixture of water and 1,4-dioxane).
Suitable acidic compounds include mineral acids (such as hydrochloric acid or sulphuric acid), organic acids (such as formic acid or acetic acid) or suitable ammonium salts (such as ammonium chloride).
In one aspect the present invention provides a process for preparing 3,5-dichloropyridine comprising reacting 2,3,5-trichloropyridine with zinc metal, in the presence of an acidic compound and at a temperature in the range 50-120xc2x0 C.
In a further aspect the present invention provides a process for preparing 3,5-dichloropyridine comprising:
a) reacting acrylonitrile with anhydrous chloral in the presence of copper (I) chloride (and preferably in the presence of a suitable solvent (such as acetonitrile)) at a temperature in the range 100-200xc2x0 C., to form 2,3,5-trichloropyridine; and,
b) reacting the 2,3,5-trichloropyridine so formed with zinc metal, in the presence of an acidic compound and at a temperature in the range 50-120xc2x0 C.
In a still further aspect steps (a) and (b) are conducted without purification of the intermediate 2,3,5-trichloropyridine.
In another aspect the present invention provides a process for the preparation of 3,5-dichloropyridine comprising reacting 2,3,4,5,6-pentachloropyridine with zinc metal, in the presence of an acidic compound and at a temperature in the range 80-120xc2x0 C., wherein a quantity of 3,5-dichloropyridine (preferably sufficient so that, under the process conditions, the melting point of 2,3,4,5,6-pentachloropyridine is below 100xc2x0 C. at the beginning of the process) is present at the beginning of the process.